Exhaust gas emitted from internal combustion engines such as vehicle engines contains harmful substances, such as hydrocarbons (HC) carbon monoxide (CO), and nitrogen oxides (NOx). Since such substances cause air pollution, purification of exhaust gas is necessary.
Exhaust gas can be purified using exhaust gas purifying catalysts. In general, exhaust gas purifying catalysts are composed of a substrate and a catalyst layer provided on the substrate. Such catalyst layer comprises a carrier and a catalyst metal supported on the carrier. Noble metals such as platinum, palladium, and rhodium are commonly used as catalyst metals.
Metals other than noble metals can also be used as catalyst metals. For example, Patent Document 1 discloses a NOx purifying catalyst using vanadium, niobium, tantalum, or tin as a catalyst metal.